Seating structure having a backrest with a flexible membrane and a moveable armrest

ABSTRACT

A seating structure includes a backrest having a top and bottom and includes a frame. The frame includes a pair of spaced apart side members defining an opening therebetween. Each of the side members has a length. A flexible membrane extends between and is attached to the side members across the opening. An armrest is moveably mounted to each of the side members. Each of the armrests is adjustably moveable along at least a portion of the length of the side members between at least a first position and a second position. The first position is closer to the bottom of the backrest than the second position. A method of adjusting the seating structure is also provided.

RELATION TO OTHER APPLICATIONS

[0001] This application is a continuation-in-part of Ser. No.07/898,907, filed Jun. 15, 1992, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to office chairs, andmore particularly, to a height-adjustable, tiltable office chair whichsupports the body of a user in ergonomically desirable positions forperforming various tasks.

[0003] Office chairs are typically configured to allow tilting of theseat and backrest as a unit or tilting of the backrest relative to theseat. In chairs having a backrest pivotally attached to a seat in aconventional manner, the movement of the backrest relative to the seatcan create shear forces acting on the legs and back of the user. Theseshear forces tend to cause an uncomfortable pulling of the user'sclothing. In an attempt to compensate for these shear forces, someoffice chairs include a backrest which pivots while the seat tilts, suchas those disclosed in U.S. Pat. Nos. 2,859,801 (Moore) and 4,429,917(Diffrient). To provide a chair which naturally conforms to the pivotalmovement of both the legs and trunk of a user between tilt positions, itis desirable to provide a chair having a seat and backrest which pivotgenerally about the axis of the hip joints of the user.

[0004] To further ensure comfortable tilting between tilt positions andenhance the comfort of a user while in a given tilt position, it isdesirable to provide a chair having a tilt mechanism with an effectivepivot point about the ankles of the user. Such an ankle tilt featuredecreases the effort required to tilt the chair, reduces the pressure ofa forward edge of the seat acting on the underside of a user's leg, andallows the feet of the user to remain flat on a floor.

[0005] Although some tilting chairs have incorporated such an ankle tiltfeature, none have comprehensively addressed the overall body postureand relative positioning of body parts for ensuring comfort andminimizing fatigue regardless how intensely a user works. In most officeenvironments, a worker performs several tasks such as writing at a desk,dictating, using the telephone, or typing at a video display terminal(VDT). Not only do such tasks vary in the inherent intensity of the workbeing performed, but an individual may also desire to increase ordecrease the work intensity of a given task. As a result, the optimalposition of the body for ensuring comfort and minimizing fatigue alsovaries. Thus, it is desirable to provide a chair which automaticallysupports the body parts of a user in ergonomically optimal positions forperforming intensive, moderate, or relaxing modes of work. It is alsodesirable to provide adjustable armrests for positioning the arms of auser in optimal locations for various tasks regardless of the size andshape of a user.

[0006] A related disadvantage of conventional office chairs is theconfiguration and material of the seat and/or backrest. Such seatstypically include single or multi-density foam padding with a coveringsuch as cloth, leather or the like. This type of seating provides adeformable cushion which conforms to the user's buttocks. However, adeformable cushion does not provide a self-adjusting support whichvaries according to the position of the user and the tilt position ofthe seat. Such seating also tends to provide insufficient aeration sinceit acts as another layer of clothing. In chairs incorporating flexiblemembranes, the membranes are typically attached directly to the frame ofa seat. Often the membrane is attached to the frame by wrapping edgeportions of the membrane around spaced apart rods which define theframe. The membrane of such a seat is difficult to repair and/or replacesince the chair would typically have to be disassembled to allow suchmaintenance. In addition, the structural requirements of such anattachment limits the shape and size of the frame and the membrane.

[0007] Typically, the seats of office chairs are supported by a singlestage telescoping column which provides for vertical adjustment of theseat. These columns include a gas spring mounted in a telescoping tubewhich is slidable within a base tube. In accordance with guidelines setby the American National Standards Institute (A.N.S.I.) and Business andInstitutional Furniture Manufacturer's Association (B.I.F.M.A.),conventional office chairs in the United States are typically adjustablefrom a seat height of about 16.0 inches from a floor to about 20.5inches from a floor. Nevertheless, it is desirable to exceed this rangeof height adjustment to account for very small or large users and toaccommodate the international population in general.

[0008] Typically, it is difficult to exceed this range of heightadjustment with seats which tilt about the knees or ankles of a user. Tooffset the moments acting on single stage support columns, pneumaticmanufacturers typically set a minimum overlapping distance of 2.95inches (75 mm) between the tubes. Because such “ankle tilt” and “kneetilt” chairs have relatively large tilt housings, it is difficult toprovide a lower minimum and higher maximum seat height while maintainingthe required overlapping distance between the tubes. These types oftilting chairs also impart a greater moment on the tubes since the pivotaxis is offset from the support column. It is therefore desirable toprovide a vertically adjustable support column having a greateroverlapping distance to permit a greater stroke which decreases theminimum height and increases the maximum height of a chair seat.

SUMMARY OF THE INVENTION

[0009] Briefly stated, the invention is directed to an office chairhaving a seat, a back and a pair of armrests which support the body of auser in ergonomically desirable positions for performing various tasks.

[0010] In one aspect of the invention, a linkage assembly is adapted toallow the seat and back to tilt downwardly and rearwardly and to allowpivotal movement of the seat about a pivot axis in substantial alignmentwith the hip joints of a user to inhibit shear forces from pulling theclothing on the body of a user.

[0011] In another aspect of the invention, the linkage assembly isadapted to allow the seat and back to tilt downwardly and rearwardlysuch that the seat pivots about an effective pivot point atsubstantially the ankle of a user having feet resting on a floor.

[0012] In yet another aspect of the invention, a vertically adjustablecolumn includes an outer guide tube, an intermediate telescoping tubeslidably positioned within the outer guide tube, and an innertelescoping tube slidably positioned within the intermediate tube. Theouter guide tube is mounted to a support stand and the inner tube has anupper portion thereof connected to a seat support member. A gas springis provided and includes a cylinder mounted within the inner tube, apiston rod extending outwardly from the cylinder and having an endconnected to a bottom portion of the outer guide member, and a controlpin assembly mounted to an upper portion of the cylinder for operableengagement with an actuator member. The piston rod is extensible betweena collapsed position in which the cylinder and inner tube aresubstantially within the intermediate tube and the intermediate tube issubstantially within the outer guide tube, and a raised position inwhich a portion of the cylinder and inner tube extend outwardly from theintermediate tube and a portion of the intermediate tube extendsoutwardly from the outer guide tube.

[0013] In another aspect of the invention, the seat and back of thechair include a frame member having an central opening therethrough anda receptacle formed around the perimeter thereof. A membrane of elasticmaterial covers the central opening and is adapted to be attached to acarrier member which fits inside the receptacle.

[0014] In yet another aspect of the invention, a pair of armrests areadapted to be mounted to the chair to allow pivotal movement thereof ina plane substantially perpendicular to the general plane of the back toaccommodate the angle at which the forearms of a user are positioned.

[0015] In a preferred embodiment of the invention, the linkage assemblyincludes a pair of first links having an end pivotally attached to thelateral portions of the seat to define a pivot axis in substantialalignment with the hip joints of a user. A pair of second links have afirst section rigidly connected to the first links and the back and asecond section angled downwardly from the first section. The secondsection terminates in an end pivotally mounted to a forward portion ofthe base member. A restraining link has one end pivotally attached to arear portion of the seat and another end pivotally attached to the basemember to limit tilting of the seat. The first links, second links, andrestraining links are configured to create an effective pivot point ofthe seat at substantially the ankles of a user having feet resting on afloor. Thus, the seat pivots about the hip axis of a user such that theangle therebetween increases as the back and seat tilt rearwardly.

[0016] The chair preferably includes a bowed section to support thelumbar region of a back of a user. The greater rearward tilting of theback relative to the seat automatically lowers the bowed section toaccommodate the changing position of the lumbar region of the user.

[0017] Preferably, the chair also includes a height adjustable, flexiblestrap member positioned horizontally across the back at approximatelythe lumbar region of the back of a user. The strap member is alsolaterally adjustable to provide a desired tension for supporting theback of the user. The armrests are preferably adapted to tilt with theback such that the angle therebetween remains constant during tilting.Preferably, the armrests are height adjustable relative to the backframe by a pawl and rack mechanism.

[0018] In addition, the membrane of elastic material is preferablypre-stretched prior to insertion of the carrier member in the receptacleto accommodate for the contour of the body of a user. The carrier memberis preferably of one-piece construction and is adapted to be press-fitin the receptacle, and the edge portion of the membrane is preferablymolded with the carrier member. The carrier member can also beconfigured to snap-fit into the frame member receptacle, and the edgeportion of the membrane can be welded to the carrier member.

[0019] The embodiments of the present invention provide significantadvantages over other tiltable office chairs. For example, in the mostpreferred embodiment, the seat and back naturally conform to the body ofa user during tilting of the chair to thereby reduce shear forces actingon the thighs and trunk of the user and minimize pressure acting on theunderside of the user's thighs at the knees. This chair alsoautomatically supports the body of the user in ergonomically desirablepositions for performing tasks of varying intensity. The user's weightis distributed evenly and shifts of the center of gravity arecompensated for to allow the user to maintain equilibrium and goodbalance. Thus, the user's body is positioned to reduce fatigue, allowproper alignment for performing various tasks, achieve maximum alertnessor relaxation, and ensure overall comfort. Moreover, the range ofvertical adjustment of the chair is increased to allow a lower minimumheight and higher maximum height than conventional office chairs.

[0020] The present invention, together with further objects andadvantages, will be best understood by reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a perspective view of a preferred embodiment of a chairhaving a backrest, a seat, and a pair of armrests.

[0022]FIG. 2 is a front view of the chair.

[0023]FIG. 3 is a right side view of the chair.

[0024]FIG. 4 is a left side view of the chair.

[0025]FIG. 5 is a rear view of the chair.

[0026]FIG. 6 is a top view of the chair.

[0027]FIG. 7 is a bottom view of the chair.

[0028]FIG. 8 is a side view of the chair showing the seat and backrestin a forward tilt position.

[0029]FIG. 9 is a side view of the chair showing the seat and backrestin a reclined tilt position.

[0030]FIG. 10 is a side view of the chair showing the seat and backrestin preferred forward, middle, and reclined tilt positions.

[0031]FIG. 11A is a fragmentary side view of the chair in a raisedposition showing a preferred embodiment of a column assembly incross-section.

[0032]FIG. 11B is a fragmentary side view of the chair in a loweredposition showing the column assembly in FIG. 11A in a collapsedposition.

[0033]FIG. 12A is a fragmentary side view of the chair in a raisedposition showing an alternative embodiment of the column assembly incross-section.

[0034]FIG. 12B is a fragmentary side view of the chair in a loweredposition showing the column assembly in FIG. 12A in a collapsedposition.

[0035]FIG. 13 is a cross-sectional view of yet another embodiment of thecolumn assembly shown in FIGS. 11A and 11B.

[0036]FIG. 14 is a top view of the chair showing the armrests in variouspivoted positions indicated by broken lines.

[0037]FIG. 15 is a fragmentary perspective view of the chair showing anexploded view of an armrest assembly, a backrest frame and a linkageassembly.

[0038]FIGS. 16 and 17 are exploded top views of an alternativeembodiment of an armrest assembly.

[0039]FIG. 18 is a cross-sectional view of the armrest assembly shown inFIGS. 16 and 17.

[0040]FIG. 19 is a fragmentary rear view of an alternative embodiment ofthe backrest and a brace member.

[0041]FIG. 20 is a cross-sectional view of the backrest and brace membershown in FIG. 19.

[0042]FIG. 21 is a front view of a tilt control mechanism with variousportions removed for clarity.

[0043]FIG. 22 is a cross-sectional view of the tilt control mechanismtaken along the line 22-22 in FIG. 21.

[0044]FIG. 23 is a top view of the tilt control mechanism shown in FIGS.21 and 22 with various portions removed for clarity.

[0045]FIGS. 24 and 25 are side views of the tilt control mechanismshowing a rearward tilt limiter mechanism.

[0046]FIGS. 25 and 26 are side views of the tilt control mechanismshowing a forward tilt limiter mechanism.

[0047]FIGS. 28 and 29 are cross-sectional views of mechanisms foractuating the tilt control mechanisms.

[0048]FIG. 30 is an exploded perspective view of the seat.

[0049]FIG. 31 is a perspective view of the seat shown in FIG. 30.

[0050]FIG. 32 is a cross-sectional view of the seat taken along the line32-32 in FIG. 31.

[0051]FIG. 33 is a cross-sectional view of the seat shown in FIG. 31.

[0052]FIG. 34 is a cross-sectional view of the seat taken along the line34-34 in FIG. 32.

[0053]FIG. 35 is a cross-sectional view of the seat shown in FIG. 31.

[0054]FIG. 36 is an exploded fragmentary view of a seat membrane.

[0055]FIGS. 37 and 38 are cross-sectional views of the membrane takenalong the lines 37-37 and 38-38 in FIG. 36.

[0056]FIG. 39 is a top view of upper and lower loom members and clampmembers clamped to the seat membrane.

[0057]FIG. 40 is a cross-sectional view of the loom members, clampmembers, and membrane taken along the line 40-40 in FIG. 39.

[0058]FIG. 41 is a top view of a lower mold member.

[0059]FIG. 42 is a top view of the loom member and membrane loaded inthe lower mold member.

[0060]FIG. 43 is a cross-sectional view of the loom member, membrane andmold members taken along the line 43-43 in FIG. 42 and showing the moldmembers prior to closure.

[0061]FIG. 44 is a cross-sectional view of the mold members in FIG. 43shown closed against the loom member and membrane.

[0062]FIG. 45 is a cross-sectional view of the loom member, membrane andmold members taken along the line 45-45 in FIG. 42 and showing the moldmembers prior to closure.

[0063]FIG. 46 is a cross-sectional view of the mold members in FIG. 45shown closed against the loom member and membrane.

[0064]FIG. 47 is an exploded cross-sectional view of the closed moldmembers in FIGS. 44 and 46 showing a cavity and the membrane.

[0065]FIG. 48 is a cross-sectional view of the mold members and membranetaken along the line 48-48 in FIG. 47.

[0066]FIG. 49 is a perspective view of an alternative embodiment of theseat.

[0067]FIG. 50 is an exploded, cross-sectional view of the seat shown inFIG. 49.

[0068]FIG. 51 is a perspective view of another alternative embodiment ofthe seat.

[0069]FIG. 52 is a cross-sectional view of the seat shown in FIG. 51.

[0070]FIG. 53 is a perspective view of another alternative embodiment ofthe seat.

[0071] FIGS. 54-56 are cross-sectional views of the seat shown in FIG.53 taken along the lines 54-54, 55-55, and 56-56 in in FIG. 26.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] Referring to the drawings, FIGS. 1-7 show a preferred embodimentof a chair 30 in a middle tilt position. The chair 30 includes a seat32, a backrest 34, and a pair of armrest assemblies 36. The seat 32 andbackrest 34 are connected to a tilt control housing 38 by a linkageassembly 40. The tilt control housing 38 is mounted on a verticallyadjustable, dual stage support column 42 which is secured to the centerof a pedestal 44. The pedestal 44 is movably supported on a floor 46 bya plurality of casters 48 or the like.

[0073] In a preferred embodiment of the invention, the linkage assembly40 includes a pair of first links 50 pivotally attached to upwardlyextending side portions 52 of a seat frame 33 at pivot points 54 todefine a pivot axis at substantially the hip joints of a user. A pair ofsecond links 56 each have a substantially straight first section 58 towhich the first links 50 are fixedly attached and a second section 60angled downwardly from the first section 58. An upwardly extending rearend portion 62 of each first section 58 is connected to a frame 64 ofthe backrest 34, and an inwardly extending front end portion 66 of eachsecond section 60 is pivotally mounted to a forward portion of the tiltcontrol housing 38. Thus, the rigidly connected first links 50 andsecond links 56 act as two bars of a four bar linkage which creates aneffective pivot point 68 at substantially the ankles of a user havingfeet resting on the floor 46.

[0074] As best shown in FIGS. 8-10, the seat 32 and backrest 34 bothpivot about the hip pivot points 54 while simultaneously tiltingrearwardly. To limit tilting of the seat 32, linkage assembly 40includes a pair of restraining links 70 which form a four bar linkage inconjunction with the first links 50 and second links 56. The restraininglinks 70 have one end 72 pivotally attached to a front portion of thetilt control housing 38 rearwardly and below the attachment of the endportions 66 of the links 56 to the housing 38. Another end 74 of therestraining links 70 is pivotally attached to a corresponding clevis 76extending downwardly from a rear edge of the seat 32. Thus, the seat 32pivots about hip pivot point 54 since it is pivotally mounted to thefirst links 50, and the backrest 34 pivots about the same pivot point 54since the second links 56 are fixedly connected to the first links 50.In addition, the seat 32 and backrest 34 simultaneously pivot aboutpivot point 66, and the restraining links 70 cause the seat 32 to pivotabout the effective pivot axis 68 at the ankles of a user.

[0075] One advantage of the foregoing chair configuration is theminimizing of shear forces acting on the clothing worn on the legs andtorso of the user as the user tilts between various positions. Since thelegs and torso of a user naturally pivot about the hip joints, and boththe seat 32 and backrest 34 pivot about the same pivot axis 54, theclothing of the user will not pull when changing tilt positions.Although such tilting of the seat 32 and backrest 34 is described inconjunction with a seat which pivots about the ankles of a user, theseat 32 and backrest 34 can be adapted to pivot about other axes. Forexample, the second links 56 can be configured such that a front endportion is pivotally mounted to the tilt housing assembly 38 directlyabove the support column 42 to provide a conventional “knuckle tilt”.The second links 56 can also be rigidly attached to the first links 50and the first links 50 can be pivotally mounted to the tilt controlhousing 38 at a desired location.

[0076] The comfort of a user while tilting between various tiltpositions is also enhanced by the ankle tilt feature of the invention.Since the seat 32 tilts rearwardly about the ankles of a user, the usercan tilt rearwardly with little effort without lifting the feet off thefloor. The configuration of the linkage assembly 40 also allows tiltingof the seat 32 such that a forward edge portion 78 moves rearwardlywithout rising a substantial amount to minimize the pressure on theunderside of a user's thighs at the knees.

[0077] Another advantage of the invention is the positioning of the bodyof the user in ergonomically desirable postures regardless of the taskbeing performed or the intensity at which a user works. To this end, theseat 32 and backrest 34 are tiltable between at least a forward, middle,and reclined tilt position corresponding to an intense, normal, andrelaxed mode of work.

[0078] To increase attentiveness and minimize fatigue in a workintensive mode, it is desirable for the chair 30 to be tilted forwardlyas shown in FIGS. 8 and 10. In this position, the seat 32 is tiltedforwardly at an angle of approximately 6° from the floor 46 and theangle between the seat 32 and backrest 34 is approximately 95°. Thistilt position supports the body of a user in a slightly leaning forwardposture in which the user's feet are flat on the floor, the anglebetween the trunk and thighs is greater than 90°, the spinal centerlineis approximately perpendicular to the floor 46, and the head of the useris either perpendicular to the floor or slightly bent downward andforward. The forward tilt position tends to elevate blood pressure whichenhances reticular formation of the eyes and causes a state of alertnessdesirable for performing work-intensive tasks. The open angle betweenthe thighs and trunk of a user also enhances breathing to furtherenhance alertness.

[0079] To ensure comfort and minimize fatigue while maintaining the bodyin an attentive posture for normal modes of work, the chair 30 istiltable to a middle position as illustrated in FIGS. 1-7 and shown inbroken lines in FIG. 10. In this position, the seat 32 is substantiallyhorizontal and the angle between the backrest and seat is approximately104°. Thus, the body of user is supported in a slightly leaned backposition in which the user's feet are flat on the floor, the anglebetween the trunk and thighs is increased, and the head of the user isperpendicular to the floor.

[0080] In a reclined tilt position (FIGS. 9 and 10), the seat 32 istilted rearwardly at an angle approximately 11° from the floor 46 andthe angle between the backrest and seat is approximately 108°. The bodyof a user is supported in a leaned back position in which the feet areflat on the floor and further away from the chair 30 to open up theangle between the calf and thighs of the user. The angle between thetrunk and thighs of a user is increased and the head of the user remainsperpendicular to the floor. The trunk of the user is also angled back tothe point where a substantial weight shift occurs away from the buttocksto the spine, thereby relieving pressure and compression on the spinaldiscs.

[0081] To obtain the foregoing positions of the seat 32 relative to thebackrest 34, the first links 50, second links 56, and restraining links70 are configured to allow the angle between the seat 32 and backrest 34to increase as the seat and backrest are tilted rearwardly (FIGS. 8-10).Preferably, the angle between the backrest 34 and the floor 46 increasesat a greater rate than the angle between the seat 32 and the floor. Toprovide a desired resistance to rearward tilting of the seat 32 andbackrest 34 about ankle pivot axis 68, and further to limit the tiltingof the first links 50 and backrest 34 about hip pivot points 54, a tiltcontrol mechanism such as a torsion or compression spring is positionedin the tilt control housing 38. The second links 56 are biased forwardlyand upwardly by the tilt control mechanism which is described in moredetail below. In addition, an adjustable rearward tilt limiter mechanismis provided to vary the maximum rearward tilting of the chair 30 and aforward tilt limiter mechanism is provided to prevent forward tilting ofthe seat 32 past the generally horizontal middle position shown in FIGS.1-7. The rearward and forward tilt limiter mechanisms will be describedin more detail below. A tilt lock mechanism can also be provided to lockthe chair 30 in the forward, middle, and reclined tilt positions. Thiscan be accomplished by locking the second links 56 and backrest 34 inthe desired tilt position which also prevents movement of the seat 32.An example of this type of tilt mechanism is disclosed in U.S. Pat. Nos.4,555,085 (Bauer et al.) and 4,099,775 (Mizelle).

[0082] The chair 30 is also height adjustable to position the body of avarious size users in ergonomically desirable positions relative to afloor and/or worksurface. The dual stage, vertically adjustable supportcolumn 42 can be incorporated in any type of chair and is not limited tothe chair 30 described herein.

[0083] In FIGS. 11A and 11B, the support column 42 includes an outerguide tube 110 mounted to the pedestal 44 such that a bottom wall 112thereof is spaced apart from the floor 46. An intermediate telescopingtube 114 is slidably positioned within the outer guide tube 110. Theintermediate tube preferably 114 has an interior shoulder 116 and anexterior shoulder 118 at the middle of the tube to define a lowersection 120 having a larger inner and outer diameter than an uppersection 122. The lower section 120 of the intermediate tube 114 slidablybears against the outer tube 110, and when locked in a desired position,the overlapping area of the outer tube 110 and intermediate tube lowersection 120 offsets any moments acting on the tubes to support a usersitting on the chair 30. To limit the upward travel of the intermediatetube 114, a retaining collar 124 is mounted to the top of the outer tube110 and slidably receives the upper section 122 of the intermediate tube114. In a raised position, the exterior shoulder 118 of the intermediatetube 114 bears against the collar 124 of the outer tube 110.

[0084] An inner telescoping tube 126 is slidably positioned within theintermediate tube 114 and has a top portion which is mounted to the tiltcontrol housing 38. The inner tube 126 slidably bears against the uppersection 122 of the intermediate tube 114, and when locked in a desiredposition, the overlapping area of the inner tube 126 and intermediatetube upper section 122 further offsets any moments acting on the tubesto support a user sitting on the chair 30. In addition, the momentacting on the tubes is minimized because an upper edge 128 of theintermediate tube 126 is closer to the tilt housing 38 than conventionalsupport columns, thus decreasing the moment arm acting on the tubes. Tolimit the upward travel of the inner tube 126, a retaining collar 130 ismounted to a bottom edge of the inner tube 126 and slidably bearsagainst the lower section 120 of the intermediate tube 114. Theretaining collar 130 also carries the intermediate tube 114 therewithwhen the inner tube 126 moves upwardly.

[0085] To adjust the vertical position of the chair, a conventional gasspring 132 including a pneumatic cylinder 134 is mounted within theinner tube 126. A piston rod 136 extends outwardly from the cylinder 134in an axial direction and has an end 138 connected to the bottom wall112 of the outer guide tube 110. A control pin 140 extends upwardly froma top wall of the cylinder 134 for operable engagement with aconventional actuator member (not shown). Preferably, the actuatormember is activated by a control knob on the end of a cable (not shown)which is housed on the first link 50. The piston rod 136 is extensiblebetween a collapsed position (FIG. 12) and a raised position (FIG. 11).In the collapsed position, the cylinder 134 and inner tube 126 aresubstantially within the intermediate tube 114 and the intermediate tube114 is substantially within the outer tube 110. In the raised position,a portion of the cylinder 134 and inner tube 126 extends outwardly fromthe intermediate tube 114 and the upper section 122 of the intermediatetube 114 extends outwardly from the outer tube 110.

[0086] Thus, the intermediate tube 114 provides an additionaloverlapping support area and decreases the moment arm which wouldotherwise act on the outer tube 110 to allow the tilt housing 38 andseat 32 to be raised to a greater height. The additional heightobtainable by the tilt housing 38 and seat 32 because of theintermediate tube 114 also decreases the required height of the outertube 110. As a result, the chair 30 can be lowered to a lower as well asa higher position than conventional chairs. Preferably, the distancebetween the floor 46 and the bottom wall 112 of the outer tube 110 isapproximately ½ inch and the height of the outer tube 110 isapproximately 8½ inches to allow the tilt housing 38 to be lowered to aheight of approximately 9 inches from the floor. Moreover, the stroke ofthe piston rod 136 is preferably about 7 inches to allow the tilthousing 38 to be raised to a height of approximately 16 inches from thefloor.

[0087] FIGS. 12A-13 illustrate alternative embodiments of the supportcolumn 42. Since these embodiments are similar to the previouslydescribed embodiment, similar parts appearing in FIGS. 12A-13 arerepresented by the same reference numerals. Referring now to FIGS. 12Aand 12B, the intermediate tube 114, preferably made of steel, issubstantially cylindrical and radially spaced from the outer tube 110and inner tube 126. To guide and support the intermediate tube 114within the outer tube 110, a bushing 124 extends radially inward from atop portion of the outer tube 110, and a lower bushing 111 extendsradially outward from a bottom portion of the intermediate tube 114.Thus, the bushing 124 slidably bears against an exterior surface 113 ofthe intermediate tube 114 and the lower bushing 111 slidably bearsagainst an interior surface 115 of the outer tube 110 when theintermediate tube 114 moves axially within the outer tube 110. When auser sits on the chair, the distance between the load-bearing bushings111 and 124 defines a moment arm which acts to offset any moments actingon the intermediate tube 114. To limit upward movement of theintermediate tube 114 within the outer tube 110, a first spacer 117,preferably in the form of an annular band, is positioned between theouter tube 110, intermediate tube 114, and bushings 111 and 124. Asillustrated in FIG. 12B, the first spacer 117 preferably fits looselybetween the tubes 110 and 114 so that it remains seated on theintermediate tube lower bushing 111 as the bushing moves downwardly withthe intermediate tube 114.

[0088] The inner telescoping tube 126 is likewise radially spaced fromthe intermediate tube 114 and is preferably cylindrical and made ofsteel. To support and guide the inner tube 126 within the intermediatetube 114, a bushing 130 extends radially outward from a bottom portionof the inner tube 126, and an upper bushing 119 extends radially inwardfrom a top portion of the intermediate tube 114. To guide the inner tube126 within the intermediate tube 114, the inner tube bushing 130slidably bears against an interior surface 121 of the intermediate tube114 and the intermediate tube upper bushing 119 slidably bears againstan exterior surface 123 of the inner tube 126. Thus, the distancebetween the load-bearing bushings 119 and 130 defines a moment arm whichacts to offset any moments acting on the inner tube 126. To limit theupward travel of the inner tube 126, a second spacer 125, preferably inthe form of an annular band, is positioned between the inner tube 126,intermediate tube 114, and bushings 130 and 119. As shown in FIG. 12A,the maximum height of the chair is limited by the height of the firstand second annular spacers 117 and 125 which bear against the bushings111, 119, 124 and 130.

[0089] Therefore, the overlapping distance between the tubes 110, 114and 126, or more particularly, the distance between the load-bearingbushings 111, 119, 124 and 130 provides more lateral support thanconventional columns. As a result, the tilt housing 38 and seat 32 canbe raised to a greater maximum height and lowered to a lower minimumheight. Moreover, the embodiment of column 42 is less costly thanconventional support columns which typically have a single telescopingtube slidably bearing against a sleeve which is mounted within a topportion of the outer tube. A relatively tight tolerance must be heldbetween the sleeve and the telescoping tube in these columns to preventany angular movement or swaying of the tube. Because the presentinvention utilizes a plurality of spaced apart, load bearing bushingspositioned at the ends of the tubes, the tendency for the intermediatetube 114 and inner tube 126 to sway is reduced, thus obviating the needfor such a tight tolerance.

[0090] Another aspect of the embodiment shown in FIGS. 12A and 12B is afrusto-conically shaped mounting member 127 attached to an upper portionof the outer tube 110. The outer surface of the mounting member 127 hasa relatively large taper and mates with a frusto-conically shaped cavity129 formed in the pedestal or support stand 44 which also has arelatively large taper. Preferably, the cavity 129 is formed in a centerportion 131 of the pedestal 44 and is defined by a hub 133 extendingdownwardly therefrom. The tapered mounting member 127 fits within anupper portion of the cavity 129, and a lower portion of the outer tube110 bears against an inner wall 135 of the cavity 129 at a lower portionthereof, which provides additional lateral support for the column 42.Typically, the outer tubes of conventional support columns have acylindrical top portion extending out of a pedestal and a slightlytapered bottom portion mounted directly to a similarly shaped cavity inthe pedestal. If the bottom portion of the outer tube does not fitexactly within the cavity when assembled to the pedestal, the slighttapers may prevent the outer tube from dropping entirely within thecavity which raises the minimum height of the seat. Thus, the slightlytapered portion of conventional outer tubes typically require a tighttolerance in order to properly fit within a cavity in the pedestal. Suchtight tolerances are difficult to maintain and costly. The mountingmember 127 and cavity 129 obviates this problem by providing a largertaper which allows the outer tube 110 to drop entirely within the cavity129. In addition, the outer tube 100 does not require as tight atolerance since the upper portion is not mounted directly to thepedestal 44.

[0091] Another difficulty with conventional support columns is that thetop portion of the outer tubes typically cannot be tapered since thestress acting on the bearing sleeves, which are typically mounted withinthe top portion of the outer tubes, tends to deform the sleeves. Asdiscussed above, such deformation is unacceptable since a tighttolerance must be held between the sleeves and the outer tubes. As aresult, the cylindrical top portions of the outer tubes often extend outof the pedestal which further reduces the minimum height of the seat.Because the column 42 does not require such tight tolerances between itscomponents, and since a plurality of load-bearing members are movablyspaced from the top portion of the outer tube 110, the frusto-conicalmounting member 127 can be mounted to the top portion of the outer tube110 without fear of deformation which could bind the tubes. As a result,the top portion of the outer tube 110 can be positioned within thecavity 129 to further reduce the minimum height of the seat 32.

[0092]FIG. 13 illustrates yet another embodiment of the support column42 which includes a spring 142 positioned within a bottom portion of theouter guide tube 110. The intermediate tube 114 does not have aninterior shoulder for engagement with a retaining bushing to raise theintermediate tube with the inner tube 126. Rather, the spring 142engages a bottom edge 144 of the intermediate tube 114 to bias theintermediate tube upwardly.

[0093] Another aspect of the invention is the height adjustable, pivotalarmrests 36. As best shown in FIG. 14, the armrests 36 are pivotal aboutaxes adjacent side edge portions of the back. The axes are positionedfor approximate alignment with the elbows of a user when the user'sforearms are resting on the armrests 36 to accommodate the angle atwhich the forearms are positioned. The armrests 36 are mounted to thebackrest frame 64 to ensure proper alignment with the forearms of a userin any tilt position.

[0094] Referring now to FIG. 15, the armrest 36 includes a base plate150 on which a pad is mounted and a support arm 152 having asubstantially horizontal top plate 154. The base plate 150 has athreaded hole 156 therein and the top plate 154 has a hole 158 thereinfor receiving a threaded fastener 160 which allows the base plate 150 topivot relative to the support arm 152. A detent member 162 extendsupwardly from the top plate 154 for operable engagement with anirregular surface on the underside of the base plate 150 (not shown).The detent member 162 is biased in a direction toward the irregularsurface on the base plate 150 by a spring or the like to lock the baseplate 150 in a desired position. The armrests 36 are preferably pivotalapproximately 20° inward and 10° outward from a vertical planesubstantially normal to the general plane of the backrest 34. Thus, thearmrests 36 are pivotal to a desired angle to ensure sufficient contactwith the forearms of a user to accommodate various size users and toensure proper alignment with various work devices such as narrowkeyboards or the like.

[0095] Again referring to FIG. 15, the support arm 152 includes a cavity164 defined by spaced apart side walls 166, an end wall 167 having aradius, and a top wall 168. A pawl 170 is positioned in the cavity 164for pivotal movement about a pin 172 which extends between the sidewalls 166 of the support arm 152. An actuation button 174 extendsdownwardly from an upper end of the pawl 170 for pivoting a lower endportion 175 of the pawl 170 into and out of engagement with a pluralityof teeth 176 extending outwardly from the side member 90 of backrestframe 64. The end portion 175 of the pawl 170 is normally biased towardthe teeth 176 by a spring or the like. The upwardly extending rear endportion 62 of the first link 56 is rigidly mounted to the side member 90of backrest frame 64 by conventional fasteners (not shown) and ispreferably configured as a sleeve member having a vertical slot 178therein for slidably receiving the pawl 170. A guide member 180 ispositioned between the sleeve member 62 and the teeth 176 of thebackrest frame side members 90. The guide member 180 has a roundedsurface 182 which slidably bears against the sleeve member 62 and a slot184 therein substantially the same size and shape as the end portion 175of the pawl 170 for receiving the end portion 175. The guide member 180also has a plurality of threaded holes 186 therein and the end wall 167of the support arm 152 has threaded holes 188 therein for receivingconventional fasteners (not shown) which slidably hold the support arm152 flush against the sleeve member 62.

[0096] In operation, the actuation button 174 is depressed to pivot thepawl 170 about pin 172 to disengage the end portion 186 from the teeth176 on the backrest frame side member 90. The support arm 152 isslidably moved to a desired height, and the actuation button 174 isreleased to engage the end portion 186 of the pawl 170 with the teeth176 and lock the support arm 152 in the new position.

[0097] FIGS. 16-18 illustrate an alternative embodiment of the armrests36. The top plate 154 of the support arm 152 has an index bushing 190extending upwardly from a forward portion thereof and a pivot bushing192 extending upwardly from a rear portion thereof. The base plate 150has a hole 194 in a rear portion thereof for receiving the pivot bushing192 and a curvilinear slot 196 therein which is positioned forwardly ofthe hole 194 for receiving the index bushing 190. The base plate 150 ispivotable about the pivot bushing 192 and the curvilinear slot 196 has aconstant radius about the pivot bushing 192. To provide intermittentstops for the base plate 150, an intermediate plate 198 is positionedbetween the base plate 150 and the top plate 154 of the support arm 152.The intermediate plate 198 is attached to an underside of the base plate150 and has a hole therein for receiving the pivot bushing 192. Theintermediate plate 198 also has an indexed slot 200 therein whichunderlies the slot 196 in the base plate 150 and receives the indexbushing 190. The indexed slot 200 has a centerline with the sameconstant radius as the slot 196. Preferably, the slot 200 is defined byan edge 201 approximating three circular portions 202, 204, and 206having a slightly larger diameter than the index bushing 190. Thecircular edge portions 202, 204 and 206 are configured to retain theindex bushing 190 yet allow it to pass from one circular portion to thenext upon the application of a lateral force on the intermediate plate198. Thus, the base plate 150 can be pivoted between three lockedpositions defined by the circular edge portions 202, 204 and 206 of theintermediate plate 198. Preferably, the index bushing 190 and circularedge portions 202, 204 and 206 are oriented to lock the base plate 150in a first position perpendicular to the general plane of the backrest34, a second position 20° inward from the first position, and a thirdposition 10° outward from the first position as shown in FIG. 14. Toprovide other angular positions of the armrests 36, the indexed slot 200can be provided with a desired number of circular edge portions atdesired angles relative to the pivot bushing 192. In addition, the baseplates 150 and cushions have a bowed portion 208 which provide acomfortable support for the user's forearms when the chair 30 is in anytilt position.

[0098] Other features of the chair 30 are provided which conform to thebody of a user between tilt positions and assist in supporting the bodyin ergonomically desirable positions. For example, the seat 32 includesa self-adjusting elastic membrane 210 and the backrest 34 includes asimilar membrane 212 for comfortably supporting the user in any tiltposition. The membranes 210 and 212 and the manner in which they areattached to the seat and backrest frames 33 and 64 will be described inmore detail below.

[0099] To support the lumbar region of a user's back, the frame 64 ofthe backrest 34 includes a bowed section 214 and the membrane 212includes a corresponding bowed section 216. Since the angle between thebackrest 34 and the floor 46 increases at a greater rate than the anglebetween the seat 32 and the floor as the chair is tilted rearwardly, thebowed sections 214 and 216 of the backrest 34 automatically movedownwardly, preferably a distance of about 1.5 inches between theforward tilt and the reclined positions, to insure proper positioning ofthe lumbar support in any tilt position.

[0100] To further adjust the positioning of the lumbar support, anadjustable brace member 218 is attached to side members 220 of thebackrest frame 64. The brace member 218 is positioned horizontallybetween the side members 220 and behind the backrest membrane 212. FIG.15 shows one embodiment of a brace member 218 which is made of aflexible material such as woven nylon or the like. The brace member 218includes hook members 222 received by a vertical slot 224 formed in theside members 220 of the backrest frame 64. To adjust the height of thebrace member 218, a conventional fastener such as VELCRO® or the like(not shown) is disengaged to decrease the tension in the brace and allowmovement of the hook members 222 within the slots 224. To reconnect thebrace member 218 in a desired location and/or laterally adjust the bracemember to obtain a desired tension, the fastener is tightened a desiredamount.

[0101]FIGS. 19 and 20 illustrate another embodiment of the brace member218. In this embodiment, the backrest frame 64 is angled relative to themembrane 212 so that a rear inner edge 226 of the frame 64 is spacedapart from the membrane 212. The brace member 218 is preferably ovalshaped and made of a relatively soft yet semi-rigid material such asrubber or the like. As shown in FIG. 20, the brace member 218 has aninner surface 228 which bears against the backrest membrane 212. Thebrace member 218 is rotatably attached to the inner edge 226 of thebackrest frame 64 to allow adjustment of the angle of the brace member218. This angular adjustment stretches the membrane 212 and provides thedesired location and amount of support for the lumbar region of a user'sback. For example, the brace member 218 can be rotated from a standardposition shown in solid lines in FIG. 20 to a different position shownin broken lines in FIG. 20. Preferably, an upper edge portion 230 and alower edge portion 232 of the brace member 218 each have a radius whichprovides a comfortable surface area supporting the user's back when thebrace member 218 is rotated to an angle relative to the membrane 212.

[0102] The brace member 218 is rotatably attached to the frame 64 by apair of swivel connectors 234, 236 which are mounted to the ends of thebrace member 218. The structure of the connectors 234 and 236 issubstantially identical and will be described particularly only withreference to the connector 234. The connector 234 has a plurality ofvertically aligned hook members 238 extending perpendicularly outwardfrom a plate 240 for grasping the frame edge 226. A pivot rod 242extends perpendicularly inward from the plate 240 and is received by aninsert 244 in a ball and socket type arrangement. The insert 244 ismounted within a cavity 246 in the brace member 218 and has a pluralityof annular ribs 248 which are received by corresponding annular groovesin the cavity to prevent axial displacement of the insert 244. The pivotrod 242 is preferably mounted within the insert 244 with sufficientfrictional engagement to require manipulation of the brace member 218 inorder to pivot the brace member.

[0103] Thus, an adjustable brace member 218 is provided which supportsthe lumbar region of a user's back a desired amount at a desiredlocation. The connectors 234 and 236 can also be adapted for releasableattachment to the frame edge 226 to allow vertical adjustment of thebrace member 218.

[0104] FIGS. 21-27 illustrate the tilt control mechanism of the presentinvention. As described above, the inwardly extending front end portions66 of links 56 are pivotally attached to a forward portion of the tiltcontrol housing 38. The ends 72 of the restraining links 70 arepivotally attached to the forward portion of the housing 38 rearwardly,and below the attachment of the end portions 66 of links 56. Preferably,the end portions 66 of links 56 are rigidly attached to a hexagonal axle250 which extends transversely through the housing 38 and is rotatablyattached to spaced apart side walls 251, 252 of the housing 38 via apair of bushings 254. Likewise, the ends 72 of the restraining links 70are rigidly attached to a transversely extending bar 256 which isrotatably attached to the side walls 251, 252 of the housing 38. Toprovide a restoring torque against the rearward tilting of the seat 32,an elastomeric torsion spring 258 is mounted to the hexagonal axle 250.The spring 258 is mounted for twisting movement about the axle 250 toresist rotation of the axle 250 when a user sits on the seat 32. Atorsion spring of this type is manufactured by the B. F. GoodrichCompany and is designated as TORSILASTIC® spring. As shown in FIGS.21-23, a bushing 260 having a hexagonal core is fixedly mounted to thehexagonal axle 250, and a molded sleeve 262 of a rubber-like elastomericmaterial is fixedly mounted to the bushing 260. An outer metal sleeve264 is fixedly attached to the elastomeric sleeve 262, and an arm 266extends radially outward from the outer sleeve 264. To provide arestoring torque against rotation of the axle 250, the arm 266 isadapted to be fixed to the housing 38. Thus, the rotation of the axle250 resulting from a user sitting on the seat 32 causes the elastomericsleeve 262 to twist which exerts a restoring torque against the axle250.

[0105] The initial restoring torque exerted by the spring 258 againstthe rotation of axle 250 can be adjusted by changing the position of theouter sleeve arm 266. To provide easy adjustment, the outer sleeve arm266 has spaced apart side portions 268 defining a recess 270 in the endof the arm 266. The side portions 268 operably engage a transverselyoriented block member 272 which is threadably attached to a screw 274.The screw 274 is mounted to a bottom wall 276 of the housing 38 andextends upwardly through the recess 270 in the arm 266. The axis of thescrew 274 is positioned generally tangentially to the outer sleeve 264,and a bevel gear 278 is attached to a top portion 280 of the screw 274.A bevel gear 282 meshes with the bevel gear 278 and has an axis whichintersects the axis of the bevel gear 278. The bevel gear 282 isattached to the end of a shaft 284 which is rotatably mounted to theside wall 251 of housing 38. The shaft 284 extends horizontally outwardfrom the side wall 251 of the housing and has a handle 286 thereon toprovide easy access for a user. In operation, the shaft 284 and gear 282are rotated a desired amount to rotate the gear 278 which in turnrotates the screw 274. Rotation of the screw 274 causes the block member272 to move linearly along the axis of the screw 274, which moves theouter sleeve arm 266 of to the desired radial location. Preferably, thegear ratio of bevel gears 278 and 282 is such that a minimal amount ofeffort is required to move the arm 266. Thus, the outer sleeve arm 266is easily moved a desired amount to vary the initial restoring torque ofthe spring 258 and thereby control the rate at which the seat 32 andback 34 tilts rearwardly when a user sits on the seat 32.

[0106] An adjustable rearward tilt limiter mechanism 290 is alsoprovided to vary the maximum rearward tilting of the seat 32 andbackrest 34. As best shown in FIG. 24, a cam member 292 and gear 294 aremounted to a rod 296 which is rotatably mounted to the side wall 252 ofhousing 38. The cam member 292 preferably has a plurality of concavesurfaces 298 formed in an outer edge 300 thereof. An arm 302 is fixedlymounted to the axle 250 and has a convex follower member 304 attached toan end thereof. The arm 302 extends rearwardly from the axle 250 suchthat the follower member 304 is in operable engagement with one of theconcave surfaces 298 of the cam member 292 when a user sits on the seat32. As viewed in FIGS. 24 and 25, the maximum clockwise rotation of theaxle 250 and therefore the maximum rearward tilt position of the seat 32and backrest 34 is determined by the position of the cam member 292. Toadjust the position of the cam member 292, a pie-shaped member 306 isrotatably attached to the side wall 252 of the housing 38. The member306 has a plurality of teeth 308 on a circular edge portion thereofwhich mesh with the gear 294. A spring 310 is attached to the pie-shapedmember 306 and the side wall 252 of the housing 38 to bias rotation ofthe member 306 in a clockwise direction. A cable 312 is attached to themember 306 opposite the spring 310 and guided within a guide member 314which is attached to the side wall 252 of the housing 38. In operation,the cable 312 is moved axially a desired amount to rotate the pie-shapedmember 306, which in turn meshes with the gear 294 to rotate the cammember 292 to a desired position. When the chair is tilted rearwardly,one of the concave surfaces 298 will act as a stop for the followermember 304 to limit the rearward tilting of the seat 32 and chair 34. Asshown in dotted lines in FIG. 25, the cam member 292 and arm 302 can berotated to lock the seat 32 and backrest 34 in a forward tilt position.

[0107] In addition, a forward tilt limiter mechanism 313 is provided toprevent forward tilting of the seat 32 past the generally horizontalmiddle position shown in FIGS. 1-7. As best shown in FIGS. 26-27, apivot member 314 is mounted to a rod 316 which is rotatably mounted tothe side wall 251 of housing 38. The pivot member 314 has forward tiltabutment surface 318 and a standard tilt abutment surface 320. An arm322 is fixedly mounted to the axle 250 and has a load bearing member 324attached to an end thereof. The arm 322 extends rearwardly from the axle250 such that the load bearing member 324 is operably engageable witheither the forward tilt abutment surface 318 or the standard tiltabutment surface 320. As viewed in FIGS. 26 and 27, the maximumclockwise rotation of the axle 250 and therefore the maximum forwardtilt position of the seat 32 and backrest 34 is determined by theposition of the pivot member 314. To actuate the pivot member 314between the standard and forward tilt positions, a cable 326 is attachedto the pivot member 314. The cable member 326 is guided within a guidemember 328 which is attached to the side wall 251 of the housing 38. Inaddition, a spring 330 is attached to the side wall 251 of the housing38 and to the pivot member 314 opposite the cable 326 to bias rotationof the pivot member 314 in a counterclockwise direction as viewed inFIGS. 26 and 27. In operation, the cable 326 is moved axially a desiredamount to rotate the pivot member 314 so that the load bearing member324 is operably engageable with the standard tilt abutment surface 320as shown in FIG. 26 or with the forward tilt abutment surface 318 asshown in FIG. 27. When the chair is unoccupied or when a user leansforward, the pivot member 314 will act as a stop for the load bearingmember 324 to limit the forward tilting of the seat 32 and chair 34.

[0108] Preferably, the cables 312 and 326 are adapted to be actuatedfrom a location near the seat frame 32 so that a user does not have tolean over to adjust the tilt adjustment mechanisms 290 and 313. As shownin FIG. 28, a handle 332 can be configured for pivotal attachment withina cavity 334 formed in one of the second links 50, the seat flame 33, orother convenient location. The cable 312 or 326 can thus be actuated bymerely pivoting the handle 332 a desired amount. Alternatively, a guidemember 336 and slot 338 can be provided to allow slidable movement of ahandle 340 to actuate the cable 312 or 326 as shown in FIG. 29.

[0109] Another feature of the chair 30 which assists in comfortablysupporting a user in ergonomically desirable positions is theconfiguration of the seat 32. As best shown in FIGS. 30-35, the frame 33of seat 32 supports the elastic membrane 210 across a central opening352. Preferably, the frame 33 is a single molded piece of glass-filledthermoplastic polyester and the membrane 210 includes a plurality ofinterwoven fibers as discussed in more detail below. To provide a rimwhich conforms to the body of a user, the side portions 52 and a rearportion 354 of frame 33 curve upwardly. To minimize pressure on theunderside of a user's thighs near the knees, especially when the chair30 is tilted rearwardly, a front portion 356 of the frame 33 curvesdownwardly. The membrane 210 has a similar downwardly curving portion357 which overlies the front portion 356 of the frame 33. To furtherreduce the pressure on the legs of a user, a cushion 358 made ofpolyurethane foam or similar material fits in a recess 360 formed in thefront portion 356 of the frame.

[0110] Preferably, the entire periphery of the membrane 210 is attachedto a one-piece carrier member 362 which is removably received by acontinuous channel 364 in the seat frame 33. The channel 364 is formedin a top surface 366 of the seat frame 33 and extends around the entireperimeter of the frame 33. Although the carrier member 362 is securelyheld by the frame 33 within the channel 364, a strip 367 is provided onthe underside of the carrier member 362 and an interlocking strip 369 isprovided on a bottom surface of the channel 364 to further secure thecarrier member 362 to the frame 33. The interlocking strips 367 and 369can be hook and loop type fasteners such as VELCRO® and can beconfigured as separate tabs spaced around the periphery of the carriermember 362 and channel 364. The carrier member 362 is preferably made ofa pliable yet semi-rigid thermoplastic polyester material such aspolybutylene terephthalate (PTB), polystyrene or glass-filledpolypropylene. The membrane 210 is preferably in-molded with the carriermember 362 as described in more detail below. Thus, the carrier member362 is deformable yet has sufficient rigidity to maintain the desiredcontour of the membrane 210 when inserted in the channel 364. To thisend, the carrier member 362 is formed with the same contour as thechannel 364 including a downwardly extending front portion similar tothe curvature of the front portion 356 of the frame 33.

[0111] In addition, a top surface 368 of the carrier member 362 isconfigured to follow the contour of the top surface 366 of the frame atany location around the perimeter. Thus, the top surface 368 of thecarrier member 362 has a variable slope which generally corresponds withthe downwardly extending front portion 356, the upwardly extending sideportions 52, and the upwardly extending rear portion 354 of the frame33. A smooth transition from the carrier member 362 to the frame 33 istherefore provided, which is especially desirable in the front portionof the seat where the legs of a user rest. To provide a smoothtransition from the membrane 210 to a front portion 370 of the carriermember 362, the periphery of the membrane 210 is attached to an upperinner corner 372 of the carrier member 362 at generally the same angleas the top surface 368 of the carrier member 362. The remaining portionof the membrane 210 is shown attached to the carrier member 362 at adifferent angle than the corresponding top surface 368 of the carriermember 362. However, it will be understood that the method for attachingthe membrane 210 to the carrier member 362 allows any desired“entry-angle” between the periphery of the membrane 210 and the carriermember 362.

[0112] As shown in FIGS. 36-38, the membrane 210 is preferably made of aplurality of elastomeric monofilaments 374 interlaced with a pluralityof strands 376 of fibrous yarn typically used in textile upholsteryweaving. The elastomeric monofilaments 374 are extruded from a blockcopolymer of polyetramethylene terephthalate polyester andpolytetramethylene ether. Preferably, this material is Hytrel® which isproduced by the E. I. DuPont DeNemours Company and has a durometer of 55on the D-scale, or more specifically, Hytrel® grade 5544 or 5556. Themonofilaments 374 are extruded by standard industry techniques which arewell known to those skilled in the art. During the extrusion process,the monofilaments 374 are annealed while under tension to orient thepolyester molecules in one direction while leaving the poly ethermolecules unaffected. This increases both the tensile strength and themodulus of elasticity of the monofilaments 374.

[0113] Preferably, the block copolymer is extruded into 2350 deniermonofilaments having the following properties: EX-120 (Eytrel 5556) LotX-2174 Properties Summary 5556 Diameter (mils) Load @ 5% Load @ 10% Load@ Elongation Set No. Min. Max. Elongation (g) Elongation (g) Break (g) @Break (%)  1 16.5 24.9 215 455 4903 117 17.2 26.2 225 477 4803 113 16.624.9 210 457 5330 129 16.0 24.2 227 480 4980 122 16.0 24.2 213 461 5058122 10 15.9 24.1 239 481 4967 125 16.6 25.1 221 455 5067 122 16.5 25.2200 428 4944 124 16.1 24.3 211 441 4921 124 16.0 24.3 220 450 5121 12820 16.6 25.1 244 486 5389 127 16.6 25.4 248 489 4958 123 17.5 26.5 233472 4958 116 16.5 25.0 229 465 4999 126 15.8 23.9 225 455 4429 102 3715.8 24.0 235 489 4835 123 15.9 24.1 246 515 4890 127 16.3 24.4 234 5135266 131 16.4 25.1 193 464 4930 122 16.4 24.8 234 513 5198 129 Average16.36 24.79 225.10 472.30 4997.30 122.60 Hi 17.50 26.50 248.00 515.005389.00 131.00 Low 15.80 23.90 193.00 428.00 4429.00 102.00

[0114] The elastomeric monofilaments 374 are the primary load-carryingmembers of the membrane 210 and preferably run laterally in the warpdirection between the side portions 52 of the seat 32 to comfortablysupport a user. The monofilaments 374 conform to the shape of a user'sbuttocks and also conform to the natural movement of the body when thechair 30 is in any tilt position. Preferably, the monofilaments 374 areprestretched between 6% and 9% elongation to maintain the desiredcontour of the membrane 210 prior to imparting a load on the membrane210. In addition, the prestretching produces the optimum conformingcharacteristics of the monofilaments 374. A plurality of elastomericmonofilaments can also run longitudinally in the weft direction betweenthe rear portion 354 and the front portion 356 of the seat 32 to providefurther support which may add to the comfort of the seat 32. Ifelastomeric monofilaments are provided in both the lateral andlongitudinal directions of the seat 32, the monofilaments in the lateraldirection can be pretensioned a desired amount and the monofilaments inthe longitudinal direction can be pretensioned a different amount toproduce the desired pressure distribution of the seat 32.

[0115] To provide greater comfort to a user, the cross-sections of theelastic monofilaments 374 preferably have a width to height ratio in therange of 1.5/1 to 2/1. This provides greater comfort because theincreased width of the monofilaments provides a greater surface area forsupporting a user which distributes the forces acting on the user. Thus,the user feels less pressure from the individual monofilaments 374 asopposed to round monofilaments which are more like concentrated pressurepoints. In addition, the greater width of the monofilaments 374 createsa more opaque appearance of the membrane 210 which is attractive and maylessen the perception that the user is sitting on a net rather than aconventional cushion. In addition, the cross-section of themonofilaments 374 are preferably elliptical as shown in FIGS. 37 and 38to provide a less abrasive support. The monofilaments can be configuredwith various other cross-sectional shapes which are less abrasive than aconventional round monofilament. To extrude the monofilaments 374 intothe desired elliptical shape, the dies through which the block ofcopolymer material is drawn can have a octogonal cross-section.Preferably, the elliptical monofilanents 374 have a width ofapproximately 0.02479 inches and a height or thickness of approximately0.01636 inches. With these dimensions, the membrane 210 has about 24-26monofilaments per inch in the lateral direction.

[0116] Referring again to FIGS. 36-38, the fiber strands 376 runlongitudinally in the weft direction of the seat 32 and are preferablyarranged in groups of three. Each strand 376 preferably includesadjacent multifilament bundles 376A and 376B of spun, textured, ortwisted 1500 denier Nylon or polyester yarn. To provide additionalsupport in the longitudinal direction of the seat 32, an elasticmonofilament 378 such as spandex is incorporated into each strand 376 byspinning, air jet texturing or covering the monofilament 378. Themonofilaments 378 are preferably Lycra® monofilaments sold by the E. I.DuPont DeNemours Company, although other materials such as Hytrel® canbe used to provide the desired support. The monofilaments 378 can besecured to the strands 376 in any suitable manner such as wrapping thefibers of bundles 376A or 376B around the monofilaments 378. Inaddition, a desired number of monofilaments 378 can be provided. Thestrands 376 are preferably prestretched between 3% and 5% elongation inorder to maintain the desired contour of the membrane 210 with no loadimparted on the membrane 210. In addition, the strands 376 are secondaryload bearing members of the seat 32 and the prestretching produces theoptimum conforming characteristics of the strands 376 when a user sitson the membrane 210. Preferably, the density of the strands 376 isapproximately 7-10 strands per inch.

[0117] As shown in FIG. 36, the strands 376 are interlaced with theelastomeric monofilaments 374 in an attractive, tightly woven patternwhich facilitates aeration and provides a smooth seating surface. Thestrands 376 are held in groups of three by pairs of the elastomericmonofilaments 374 which cross over between each group of strands. Forexample, monofilaments 374A and 374B are shown in FIG. 36 crossing overbetween a group 380 and a group 382 of strands 376. To maintain thespacing between each strand 376 in a group, the monofilaments 374 weavealternately above and below adjacent strands in the group. The pluralityof strands 376 provide a relatively large surface area of nonabrasivefabric which distributes the forces acting on a user to avoid a“grid-mark” type feel resulting from the concentration of pressure. Inaddition, the weave pattern provides sufficient aeration through theopenings between the monofilaments 374 and the strands 376 to allowevaporation of perspiration and facilitate air circulation to minimizeheat buildup. The longitudinal orientation and the grouping of thestrands 376 also provide an attractive seat with a longitudinal designand a relatively opaque appearance.

[0118] The method for forming the carrier member 362 and attaching themembrane 210 thereto will now be described with reference to FIGS.39-48. As shown in FIGS. 39 and 40, a loom 384 having an upper member386 and a lower member 388 is provided to capture and hold the membrane210 in a stretched condition. The lower member 388 of loom 384 isinitially placed in a stretching machine (not shown). The membrane 210with the previously described weave pattern is then placed over thelower loom member 388, and clamp members 390 of the stretching machineclamp the edges of the membrane 210 and stretch it a predeterminedamount in both the lateral and longitudinal directions. The upper loommember 386 is then clamped against the lower loom member 388 to hold themembrane 210 in the stretched condition. The clamp members 390 of thestretching machine release the membrane 210, and excess edge portions392 of the membrane 210 outside the loom are trimmed a desired amount.

[0119] The loom 384 has a semi-rectangular shape with a central openinglarger than the area defined by the carrier member 374. The loom 384also has a downwardly extending front portion 394 which is similar tothe contour of the downwardly extending front portion 356 of the frame33. The upper and lower loom members 386 and 388 each include afrusto-conically shaped cover member 396 made of plastic such as anepoxy, urethane, or other suitable soft material which is molded over abent steel tube 398. The steel tubes 398 are provided to give the loommembers 386 and 388 structural support and the plastic members 396 areprovided protect a steel molding tool from chipping or other damage. Theupper loom member 386 has a pair of ridges 400 extending downwardlytherefrom and the lower loom member 388 has a pair of matching grooves402 formed therein to secure the membrane 210 therebetween. A pluralityof clamp devices (not shown) are also provided to hold the loom memberstogether and maintain the membrane 210 in the stretched condition.

[0120] As best shown in FIGS. 41-46, a molding tool 404 includes anupper mold member 406 and a lower mold member 408. The upper and lowermold members 406 and 408 have corresponding recesses 410 and 412 whichare configured to receive the upper and lower loom members 386 and 388.The mold members 406 and 408 also have upper and lower recesses 414 and416 which form a cavity 418 when the mold members 406 and 408 are closedtogether. The cavity 418 has the desired shape and contour of thecarrier member 362 and a substantial portion of the cavity 418 is thesame shape and contour of the channel a14 in the seat frame 33.

[0121] After the clamp members 390 of the stretching machine are removedand the excess edge portions of the membrane 210 are trimmed, theassembly of the loom 384 and stretched membrane 210 is removed forplacement in the lower mold member 408. The lower loom member 388 isthen placed in the recess 412 in the lower mold member 406 such that themembrane 210 is shaped over a curved inner male portion 420 of the lowermold member 408 as shown in FIG. 43. Thus, a periphery 422 of themembrane 210 is positioned at the desired angle over the recess 412 inthe lower mold 408 and the prestretched membrane 210 attains the desiredcontour prior to closing the upper mold 406 against the lower mold 408.The upper mold 406 is then closed against the lower mold 408 withoutfurther stretching the membrane 210 or changing the position of theperiphery 422 of the membrane 210 over the recess 412. The loom recess410 in the upper mold 406 receives the upper loom member 386, and afemale inner curved portion 424 of the upper mold 406 which is the samecontour as the male inner curved portion 420 of the lower male mold 408bears against the membrane 210. A plastic resin is then injected intothe cavity 418 to secure the periphery 422 of the membrane 210. Theupper and lower mold members 406 and 408 are then pulled apart, thecarrier member 362 and membrane 210 assembly are removed, excess edgeportions outside the carrier member 362 are trimmed, and theinterlocking strip 367 is adhesively bonded to the underside of thecarrier frame 362.

[0122]FIGS. 47 and 48 illustrate the upper and lower mold members 406 ina closed or “shut-off” position during which the resin is injected intothe cavity 418. Because the membrane 210 has a thickness, the moldmembers 406 and 408 cannot be completely clamped against each other. Theweaving of the monofilaments 374 and strands 376 creates a variablethickness membrane 210, and the closest the mold members 406 and 408 canbe clamped together is determined by the thickest portions of themembrane 210. As shown in FIGS. 47 and 48, the thickest portions of themembrane 210 are in the regions where the elastomeric monofilaments 374cross-over between the groups of strands 376. When the mold members 406and 408 are closed against the monofilaments 374 in the cross-overregion, the strands 376 and the single monofilaments 374 which are notoverlapping have less thickness, thus creating a gap 426 between themold members 406 and 408. It is desirable to minimize the gap 426 toinhibit the resin from leaking out of the cavity 418. Because theoverall thickness of the overlapping elliptical monofilaments 374 isless than the thickness of overlapping round monofilaments, the moldmembers 406 and 408 can be clamped closer together to minimize the gap426 and decrease the amount of leakage. In addition, the reduction inoverall thickness tends creates less stress when the mold members 406and 408 are clamped against the membrane 210 which tends to cause lessdamage to the monofilaments 374 and strands 376.

[0123] FIGS. 49-56 illustrate alternative embodiments of the seat 32.Since these embodiments are similar to the previously describedembodiment, similar parts appearing in FIGS. 49-56 are represented bythe same reference numerals. In FIGS. 49-56, the seat frame 33 includesa plurality of spaced apart slots 430 formed therein which extendthrough a bottom wall 432 of the channel a14. The carrier member 362 hasa plurality of spaced apart tabs 434 extending downwardly fromtherefrom. The tabs 434 have a hook portion 436 extending outwardly froma lower end thereof and the slots 430 are of sufficient size to receivethe tabs 434. The carrier member 362 is secured to the seat frame 33 byinserting the carrier member into the channel a14 and snapping the tabs434 into the slots 430 such that the hook portions 436 extend throughthe slots 430 and engage an underside 438 of the frame 33. Thisembodiment for attaching the carrier member 374 to the frame 33 can beused with any type of seating utilizing a carrier frame regardless ofthe method used to attach the seating surface to the carrier member.

[0124] FIGS. 49-56 also illustrate alternative embodiments for attachingthe membrane 210 to the carrier frame 362. For example, the periphery422 of the membrane 210 can be molded with an insert 440 which is pressfit inside a cavity 442 formed in a top surface of the carrier member362 (FIGS. 49-50). The periphery 422 of the membrane 210 can also bevibration or sonic welded to a bottom surface 444 of the carrier member362 as shown in FIGS. 51-56, and a plurality of teeth members 446 canextend downwardly from the bottom surface 444 to further secure themembrane 210 thereto as shown in FIG. 52.

[0125] Preferably, the backrest 34 is constructed with the samematerials and in the same manner as the seat 32, although the desiredamount of prestretching of the elastomeric monofilaments and strands mayvary to reflect the different support required for the back of a user.

[0126] Pre-assembly of the seat and backrest membranes to the carriermembers facilitates maintenance since the membrane/carrier memberassemblies can be easily removed for repair and/or replacement. Thisconfiguration also provides greater manufacturing flexibility. Since thecarrier members do not have the structural requirements of a seat orbackrest frame, a desired material can be used for a desired type ofattachment method. For example, the material of the carrier member canbe chosen based on whether the membrane is to be insert molded therewithor welded thereto. A semi-rigid material can be used to facilitate theinsert molding process and pre-stretching of the membrane, and amaterial having a low melting point can be used to facilitate welding ofthe membrane thereto.

[0127] The in-molding process for attaching the membranes to the carrierframes also provides significant advantages over other attachmentmethods. For example, the configuration of the mold cavities can bevaried to provide the frames and carrier members with any type ofcontour which facilitates design flexibility. The top surfaces of theframes and carrier members can have the same slope to provide a smoothtransition between the frames and the carrier members. The shape of themembranes and the angle at which the membranes are attached to thecarrier members can also be easily adjusted. In addition, the in-moldingprocess allows the frames to be thinner because a relatively smallchannel is all that is required for attachment of the carrier members tothe frame.

[0128] The membranes also provide a flexible support which conforms tothe natural movement of the body of a user when the chair is in any tiltposition. FIGS. 2-10 show the approximate position of the seat andbackrest membranes when a user is sitting thereon. The membranes tend tominimize fatigue because they are responsive to micro-postural changesof a user which stimulates muscles in the trunk and allows spinalmovement to hydrate the spinal discs.

[0129] In the forward tilt position (FIGS. 8 and 10), the seat membrane210 maintains a forward angle of the pelvis which insures a propercurvature of the spine. The elasticity of the seat membrane 210 allowsthe thighs of a user to slant forward while keeping the pelvis on ahorizontal plane, thereby giving the user the sensation of not slidingout of the chair and reducing shear forces acting on the underside ofthe user's thighs. The seat membrane 210 also passively positions thelumbar region of a user's back against the bowed section 86 of thebackrest membrane 82. In the middle and reclined tilt positions shown inFIGS. 2-7 and 9, the elasticity of the seat membrane 210 automaticallycauses larger buttocks to wedge more deeply into the pocket between theseat 32 and backrest 34 to insure correct positioning of the user'slumbar region against the bowed section 86 of the backrest mat.

[0130] The porosity of the backrest and seat membranes allows air toflow through the membranes to aerate the skin of a user. Providing suchaeration decreases uncomfortable heat buildup which would otherwiseoccur when a user sits for an extended period of time on conventionalchair upholstery which acts like another layer of clothing.

[0131] Thus, the tilt motion of the chair 32 and the resiliency of theseat and backrest membranes passively stabilizes the pelvic-lumbarprocess to reduce muscle activity heretofore associated with legcrossing and slumping postures. In addition, the membranes accommodatefor angular variations in the sacral plates of various user's spinalcolumns.

[0132] Thus, in accordance with the most preferred embodiment, a chairis provided which naturally conforms to the body of user during tiltingof the chair to reduce shear forces acting on the thighs and trunk ofthe user and minimize pressure acting on the underside of the user'sthighs at the knees. The chair automatically supports the body of theuser in ergonomically desirable positions for performing tasks ofvarying intensity, and the range of vertical adjustment of the chairallows a lower minimum height and higher maximum height thanconventional office chairs.

[0133] Although the present invention has been described with referenceto preferred embodiments, those skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. As such, it is intended that the foregoingdetailed description be regarded as illustrative rather than limiting.It is the appended claims, including all equivalents thereof, which areintended to define the scope of the invention.

We claim:
 1. A tiltable chair comprising: a base member; a seat; a back;and a linkage assembly connecting the seat and back to the base member,said linkage assembly adapted to allow the seat and back to tiltdownwardly and rearwardly and to allow pivotal movement of the seatabout a pivot axis substantially in alignment with the hip joints of auser to reduce shear forces.
 2. The chair of claim 1 wherein the back isrigidly connected to a pair of first links adapted to be pivotallymounted to the base member, each first link having an end pivotallyattached to a lateral portion of the seat at said hip pivot axis of theuser, whereby rearward tilting by a user causes the seat to pivot aboutthe hip pivot axis and also causes tilting movement of the first linksrelative to the base member such that the seat and back tilt downwardlyand rearwardly.
 3. The chair of claim 2 wherein the back is rigidlyconnected to the first links by a pair of second links, said secondlinks having an end pivotally mounted to the base member to provide thetilting movement of the first links, seat and back.
 4. The chair ofclaim 3 wherein the ends of the second links are pivotally mounted to aforward portion of the base member and a forward section of the secondlinks extends upwardly and rearwardly from said ends to create aneffective pivot point at substantially the ankles of a user having feetresting on a floor, whereby the seat tilts downwardly and rearwardlyabout said ankle pivot point such that a forward edge portion thereofmoves rearwardly without rising a substantial amount.
 5. The chair ofclaim 2 further comprising a restraining link having one end pivotallyattached to a rear portion of the seat and another end pivotallyattached to the base member to limit tilting of the seat.
 6. The chairof claim 1 wherein the linkage assembly is configured such that theangle between the back and a horizontal plane increases at a greaterrate than the angle between the seat and a horizontal plane as a usertilts rearwardly.
 7. The chair of claim 6 wherein the back includes abowed section to support the lumbar region of a back of a user and thegreater rearward tilting of the back relative to the seat automaticallylowers the bowed section to accommodate the changing position of thelumbar region of the user.
 8. The chair of claim 1 wherein the seatcomprises a membrane of elastic material pre-stretched across a centralopening in a frame member to provide a support which conforms to thebody of a user when the chair is in any tilt position.
 9. The chair ofclaim 1 wherein the back comprises a membrane of elastic materialpre-stretched across a central opening in a frame member to provide asupport which conforms to the body of a user when the chair is in anytilt position.
 10. The chair of claim 9 further comprising a bracemember positioned horizontally across the back at approximately thelumbar region of a back of a user, said brace member being adjustable toprovide a desired pressure against the membrane to support the back ofthe user.
 11. The chair of claim 10 wherein the brace member is heightadjustable.
 12. The chair of claim 1 further comprising a pair ofarmrests adapted to tilt with the back such that the relative angletherebetween remains constant during tilting, the armrests also adaptedto pivot laterally in a plane substantially perpendicular to the back toaccommodate the angle at which the forearms of a user are positioned.13. The chair of claim 12 wherein each armrest is pivotal about an axisadjacent a corresponding side edge portion of the back, said axes beingpositioned for approximate alignment with the elbows of a user havingforearms resting on the armrests.
 14. The chair of claim 13 wherein thearmrests are height adjustable.
 15. A tiltable chair comprising: a basemember; a seat; a back; and a linkage assembly between the base member,seat and back, said linkage assembly adapted to allow the seat and backto tilt downwardly and rearwardly such that the seat pivots about aneffective pivot point at substantially the ankles of a user having feetresting on a floor, said linkage assembly also adapted to allow one ofthe back and the seat to tilt relative to the other about a pivot axisin substantial alignment with the hip joints of a user such that theangle between the back and seat increases as the seat and back pivotrearwardly about the ankle of a user.
 16. The chair of claim 15 whereinthe linkage assembly is configured such that the angle between the backand a horizontal plane increases at a greater rate than the anglebetween the seat and a horizontal plane as the chair is tilted by auser.
 17. The chair of claim 16 wherein the seat and chair are tiltablebetween a forward position in which the seat is tilted forwardly at anangle from a horizontal plane and the angle between the seat and back isgreater than 95°, a middle position in which the seat is substantiallyhorizontal and the angle between the back and seat is approximately104°, and a reclined position in which the seat is tilted rearwardly atan angle approximately 11° from a horizontal plane and the angle betweenthe back and seat is approximately 108°.
 18. The chair of claim 16further comprising a restraining link having one end pivotally attachedto a rear portion of the seat and another end pivotally attached to thebase member to limit the rate at which the seat tilts.
 19. The chair ofclaim 16 wherein the linkage assembly is adapted to allow pivotalmovement of both the back and seat about the hip pivot axis as the seatand back tilt downwardly and rearwardly.
 20. The chair of claim 15wherein the linkage assembly includes a pair of first links fixedlyattached to a pair of second links and having an end pivotally attachedto a lateral portion of the seat at said hip pivot axis of the user,each second link having a forward portion extending upwardly andrearwardly from an end which is pivotally mounted to a forward portionof a base member to create the effective pivot point of the seat atsubstantially the ankles of a user.
 21. The chair of claim 15 whereinthe back includes a bowed section to support the lumbar region of a backof a user and the greater rearward tilting of the back relative to theseat automatically lowers the bowed section to accommodate the changingposition of the lumbar region of the user.
 22. The chair of claim 15wherein the seat comprises a membrane of elastic material pre-stretchedacross a central opening in a frame member to provide a support whichconforms to the body of a user when the chair is in any tilt position.23. The chair of claim 22 wherein the back comprises a membrane ofelastic material pre-stretched across a central opening in a framemember to provide a support which conforms to the body of a user whenthe chair is in any tilt position.
 24. The chair of claim 23 furthercomprising a brace member positioned horizontally across the back atapproximately the lumbar region of a back of a user, said brace memberbeing adjustable to provide a desired pressure against the membrane tosupport the back of the user.
 25. The chair of claim 24 wherein thebrace member is height adjustable.
 26. The chair of claim 15 furthercomprising a pair of armrests adapted to tilt with the back such thatthe relative angle therebetween remains constant during tilting, thearmrests also adapted to pivot laterally in a plane substantiallyperpendicular to the back to accommodate the angle at which the forearmsof a user are positioned.
 27. The chair of claim 26 wherein each armrestis pivotal about an axis adjacent a corresponding side edge portion ofthe back, said axes being positioned for approximate alignment with theelbows of a user having forearms resting on the armrests.
 28. The chairof claim 27 wherein the armrests are height adjustable.
 29. A tiltablechair comprising: a base member; a seat having front, rear and lateraledge portions; a back; a pair of first links having an end pivotallyattached to the lateral portions of the seat in alignment with a pivotaxis at substantially the hip joint of a user; a pair of second linkshaving a first section fixedly connected to the first links and the backand a second section angled downwardly from the first section, saidsecond section terminating in an end pivotally mounted to a forwardportion of the base member; and a pair of restraining links having oneend pivotally attached to a rear portion of the seat and another endpivotally attached to the base member to limit tilting of the seat;whereby the first links, second links, and restraining links create aneffective pivot point of the seat at substantially the ankles of a userhaving feet resting on a floor and the seat pivots about the hip axis ofa user to increase the angle therebetween as the back and seat tiltrearwardly.
 30. The chair of claim 29 wherein the seat and chair aretiltable between a forward position in which the seat is tiltedforwardly at an angle from a horizontal plane and the angle between theseat and back is greater than 95°, a middle position in which the seatis substantially horizontal and the angle between the back and seat isapproximately 104°, and a reclined position in which the seat is tiltedrearwardly at an angle approximately 11° from a horizontal plane and theangle between the back and seat is approximately 108°.
 31. The chair ofclaim 29 wherein the back includes a bowed section to support the lumbarregion of a back of a user and the greater rearward tilting of the backrelative to the seat automatically lowers the bowed section toaccommodate the changing position of the lumbar region of the user. 32.The chair of claim 29 wherein the seat comprises a membrane of elasticmaterial pre-stretched across a central opening in a frame member toprovide a support which conforms to the body of a user when the chair isin any tilt position.
 33. The chair of claim 29 wherein the backcomprises a membrane of elastic material pre-stretched across a centralopening in a frame member to provide a support which conforms to thebody of a user when the chair is in any tilt position.
 34. The chair ofclaim 33 further comprising a height adjustable brace member positionedhorizontally across the back at approximately the lumbar region of aback of a user, said brace member being adjustable to provide a desiredpressure against the membrane to support the back of the user.
 35. Thechair of claim 29 further comprising a pair of height adjustablearmrests adapted to pivot laterally in a plane substantiallyperpendicular to the back to accommodate the angle at which the forearmsof a user are positioned.
 36. A tiltable chair comprising: a seat havinga membrane of elastic material pre-stretched across a central opening ina frame member to provide a support which conforms to the body of a userwhen the chair is in any tilt position; a back having a membrane ofelastic material pre-stretched across a central opening in a framemember to provide a support which conforms to the body of a user whenthe chair is in any tilt position, said frame member having a bowedsection to support the lumbar region of a back of a user a heightadjustable brace member positioned horizontally across the back atapproximately the lumbar region of a back of a user, said brace memberbeing adjustable to provide a desired pressure against the membrane tosupport the back of the user; a pair of height adjustable armrestsadapted to tilt with the back such that the angle therebetween remainsconstant during tilting, said armrests also adapted to laterally pivotin a plane substantially perpendicular to the back about axes adjacentside edge portions of the back, said axes being positioned forapproximate alignment with the elbows of a user having forearms restingon the armrests; and a linkage assembly adapted to allow the seat andback to tilt downwardly and rearwardly such that the seat pivots aboutan effective pivot point at substantially the ankle of a user havingfeet resting on a floor, said linkage assembly also adapted to allowpivotal movement of the seat about a pivot axis at substantially the hipjoints of a user such that the angle between the back and a horizontalplane increases at a greater rate than the angle between the seat and ahorizontal plane as a user pivots the back and seat, said pivotalmovement of the back relative to the seat automatically moving the bowedsection of the frame in a generally vertical direction to accommodatethe changing position of the lumbar region of the user.
 37. A verticallyadjustable column for supporting the seat of a chair, the columncomprising: an outer guide tube mounted to a support stand; anintermediate telescoping tube slidably positioned within the outer guidetube; an inner telescoping tube slidably positioned within theintermediate tube and having an upper portion thereof connected to aseat support member; and a gas spring including a cylinder mountedwithin the inner tube, a piston rod extending outwardly from thecylinder in an axial direction and having an end connected to a bottomportion of the outer guide member, and a control pin assembly mounted toan upper portion of the cylinder for operable engagement with anactuator member, said piston rod being extensible between a collapsedposition in which the cylinder and inner tube are substantially withinthe intermediate tube and the intermediate tube is substantially withinthe outer guide tube, and a raised position in which a portion of thecylinder and inner tube extend outwardly from the intermediate tube anda portion of the intermediate tube extends outwardly from the outerguide tube.
 38. The column of claim 37 wherein approximately one halfthe length of the inner tube is within the intermediate tube andapproximately one half the length of the intermediate tube is within theouter guide tube when the piston rod is extended to the raised position.39. The column of claim 37 further comprising a spring positioned withina bottom portion of the outer guide tube, said spring engaging theintermediate tube to bias said intermediate tube in an upward direction.40. The column of claim 37 further comprising first retaining means forlimiting the upward travel of the intermediate tube within the outerguide tube and second retaining means for limiting the upward travel ofthe inner tube within the intermediate tube.
 41. The column of claim 40wherein the first retaining means comprises an outer collar mounted to atop edge portion of the outer guide tube for engagement with an exteriorshoulder of the intermediate tube, and the second retaining meanscomprises an inner collar mounted to a bottom edge portion of the innertube for engagement with an interior shoulder of the intermediate tube.42. The column of claim 41 wherein the exterior shoulder and interiorshoulder of the intermediate tube are at substantially the middle of thelength of said intermediate tube.
 43. The column of claim 39 wherein thedistance between a floor and a bottom of the outer guide tube isapproximately one half inch and the height of the outer guide tube isapproximately 8½ inches to allow the seat support member to be loweredto a height of approximately 9 inches from the floor, and the stroke ofthe piston rod is approximately 7 inches to allow the seat supportmember to be raised to a height of approximately 16 inches from thefloor.
 44. The column of claim 37 further comprising a frusto-conicallyshaped mounting member attached to the outer guide tube and mating witha frusto-conically shaped cavity formed in the support stand.
 45. Thecolumn of claim 44 wherein the mounting member is attached to an upperportion of the outer guide tube and fits within an upper portion of saidcavity, and a lower portion of the guide tube bears against an innerwall of the cavity at a lower portion thereof to securely mount thecolumn to the support stand.
 46. The column of claim 45 wherein thesupport stand includes a plurality of legs extending radially outwardfrom a center portion thereof, said cavity being formed in said centerportion and being further defined by a hub extending downwardly from thejuncture of the legs, thereby providing lateral support for the bottomof the outer tube to allow said outer tube to be mounted in a lowposition relative to a floor.
 47. A vertically adjustable column forsupporting a chair, the column comprising: an outer guide tube mountedto a support stand and having a collar mounted to a top portion thereof,the inner diameter of said collar being smaller than the inner diameterof the outer guide tube; an intermediate telescoping tube slidablypositioned within the outer guide tube, said intermediate tube having aninterior shoulder and an exterior shoulder, the exterior shoulderbearing against the collar of the outer tube to limit the upward travelof the intermediate tube; an inner telescoping tube slidably positionedwithin the intermediate tube and having an upper portion thereofconnected to a seat support member, said inner tube having a collarmounted to a bottom portion thereof which bears against the interiorshoulder of the intermediate tube to limit the upward travel of theinner tube; and a gas spring including a cylinder mounted within theinner tube, a piston rod extending outwardly from the cylinder in anaxial direction and having an end connected to a bottom portion of theouter guide member, and a control pin assembly mounted to an upperportion of the cylinder for operable engagement with an actuator member,said piston rod being extensible between a collapsed position in whichthe cylinder and inner tube are substantially within the intermediatetube and the intermediate tube is substantially within the outer guidetube, and a raised position in which a portion of the cylinder and innertube extend outwardly from the intermediate tube and a portion of theintermediate tube extends outwardly from the outer guide tube.
 48. Thecolumn of claim 47 wherein the exterior shoulder of the intermediatetube is at substantially the middle of the length of said tube so thatapproximately half the length thereof is within the outer guide tubewhen the piston rod is extended to the raised position, and the interiorshoulder of the intermediate tube is at substantially the middle of thelength of said tube so that approximately half the length of the innertube is within the outer guide tube when the piston rod is extended tothe raised position.
 49. The column of claim 48 wherein the distancebetween a floor and a bottom of the outer guide tube is approximatelyone half inch and the height of the outer guide tube is approximately 8½inches to allow the seat support member to be lowered to a height ofapproximately 9 inches from the floor, and the stroke of the piston rodis approximately 7 inches to allow the seat support member to be raisedto a height of approximately 16 inches from the floor.
 50. The column ofclaim 47 further comprising a spring positioned within a bottom portionof the outer guide tube, said spring engaging the intermediate tube tobias said intermediate tube in an upward direction.
 51. A verticallyadjustable column for supporting a chair, the column comprising: anouter tube mounted to a support stand and having a bushing attached to atop portion thereof, the inner diameter of said bushing being smallerthan the inner diameter of the outer tube; an intermediate telescopingtube slidably positioned within the outer tube and being radially spacedtherefrom, said intermediate tube having a lower bushing mounted to alower portion thereof and an upper bushing mounted to an upper portionthereof, said outer tube bushing slidably bearing against theintermediate tube and said lower bushing of the intermediate tubeslidably bearing against the outer tube; a first spacer positionedbetween the outer tube and intermediate tube and the outer tube bushingand intermediate tube lower bushing, whereby the first spacer bearsagainst said bushings to limit upward travel of the intermediate tubewithin the outer tube; an inner telescoping tube positioned within theintermediate tube and being radially spaced therefrom, said inner tubehaving a bushing mounted to a lower portion thereof and a seat supportmember connected to an upper portion thereof, said inner tube bushingslidably bearing against the intermediate tube and said upper bushing ofthe intermediate tube slidably bearing against the inner tube; a secondspacer positioned between the inner tube and intermediate tube and theintermediate tube upper bushing and inner tube bushing, whereby thesecond spacer bears against the inner tube bushing and intermediate tubeupper bushing to limit upward travel of the inner tube within theintermediate tube; and a gas spring including a cylinder mounted withinthe inner tube, a piston rod extending outwardly from the cylinder in anaxial direction and having an end connected to a bottom portion of theouter guide member, and a control pin assembly mounted to an upperportion of the cylinder for operable engagement with an actuator member,said piston rod being extensible between a collapsed position in whichthe cylinder and inner tube are substantially within the intermediatetube and the intermediate tube is substantially within the outer guidetube, and a raised position in which a portion of the cylinder and innertube extend outwardly from the intermediate tube and a portion of theintermediate tube extends outwardly from the outer guide tube.
 52. Thecolumn of claim 47 wherein the first and second spacers are annularbands.
 53. The column of claim 47 further comprising a frusto-conicallyshaped mounting member attached to an upper portion of the outer tubeand mating with an upper portion of a frusto-conically shaped cavityformed in the support stand, a lower portion of the guide tube bearingagainst an inner wall of the cavity at a lower portion thereof tosecurely mount the column to the support stand.
 54. A chair comprising:a pair of armrests adapted to be mounted to the chair to allow pivotalmovement in a lateral direction to accommodate the angle at which theforearms of a user are positioned on the armrests.
 55. The chair ofclaim 54 wherein each armrest is pivotal about an axis adjacent acorresponding side edge portion of the back, said axes being positionedfor approximate alignment with the elbows of a user having forearmsresting on the armrests.
 56. The chair of claim 55 wherein the armrestsare pivotally mounted to a pair of support arms, each support arm havinga detent member in operable engagement with an irregular surface of thearmrests, said detent members being biased in a direction toward saidirregular surfaces to lock the armrests in a desired position.
 57. Thechair of claim 55 wherein the armrests are adapted to be locked intopredetermined pivotal positions.
 58. The chair of claim 54 wherein thearmrests are height adjustable.
 59. The chair of claim 58 wherein thearmrests are height adjustable relative to a back frame of the chair bya pawl and rack mechanism.
 60. A chair comprising: a back frameincluding side members having a plurality of vertically aligned teethextending outward therefrom; a seat carrier member connected to the backframe and having a pair of vertically extending sleeve members adaptedto mate with the side members of the back frame, said sleeve membershaving a vertical slot therein; a pair of support arms slidably attachedto the seat carrier sleeve members, said support arms having a pawlpivotally mounted thereto which extends through the slot in the seatcarrier and is biased in a direction toward the back frame forengagement with the teeth, whereby a user pivots the pawl out ofengagement with the teeth to move the support arm in a generallyvertical direction and releases the pawl to lock the support arm at adesired height; and a pair of armrests pivotally mounted to the supportarms, each support arm having a detent member in operable engagementwith an irregular surface of the armrests, said detent members beingbiased in a direction toward said irregular surfaces to lock thearmrests in a desired position.
 61. A support assembly for supportingthe body of a user in a chair, the support assembly comprising: a framemember having an central opening therethrough and a channel formedaround the perimeter thereof; a carrier member adapted to fit inside thechannel; and a membrane of elastic material covering the central openingand adapted to be attached to the carrier member.
 62. The supportassembly of claim 61 wherein said membrane is pre-stretched prior toinsertion of the carrier member in the channel to accommodate for thecontour of the body of a user.
 63. The support assembly of claim 62wherein the support assembly defines a seat and said membrane ispre-stretched in a longitudinal direction between a forward and rearportion of the carrier member in the range of 6% to 9% of an unstretchedlength of the mat, and said membrane is pre-stretched in a lateraldirection between side portions of the carrier member in the range of 1%to 2% of an unstretched width of the mat.
 64. The support assembly ofclaim 62 wherein the carrier member is of one-piece construction anddeformable.
 65. The support assembly of claim 61 wherein the carriermember is press fit into the frame member channel.
 66. The supportassembly of claim 65 wherein the carrier member is made of a resilientmaterial.
 67. The support assembly of claim 66 wherein an edge portionof said membrane is molded with the carrier member.
 68. The supportassembly of claim 61 wherein the carrier member is adapted to besnap-fit into the frame member channel.
 69. The support assembly ofclaim 68 wherein an edge portion of said membrane is welded to thecarrier member.
 70. The support assembly of claim 68 wherein an edgeportion of said membrane is attached to an insert adapted to fit in acavity formed in said carrier member.
 71. The support assembly of claim70 wherein said edge portion of said membrane is molded with the insert.72. The support assembly of claim 70 wherein the edge portion of saidmembrane is welded to the insert.
 73. A support assembly for supportingthe body of a user in a chair, the support assembly comprising: a framemember having an central opening therethrough and a channel formedaround the perimeter thereof; a one-piece deformable carrier memberadapted to fit inside the channel; and a membrane of elastic materialcovering the central opening, said membrane having an edge portionattached to the carrier member and said carrier member being expanded adesired amount prior insertion into the channel to pre-stretch saidmembrane a desired amount.
 74. The support assembly of claim 73 whereinthe carrier member is press fit into the frame member channel.
 75. Thesupport assembly of claim 74 wherein the edge portion of said membraneis molded with the carrier member.
 76. The support assembly of claim 73wherein the carrier member has sufficient rigidity to maintain a shapesubstantially the same as the channel after expansion thereof to allowsaid carrier member to be snap-fit into the frame member channel. 77.The support assembly of claim 76 wherein the edge portion of saidmembrane is attached to an insert adapted to fit in a cavity formed insaid carrier member.
 78. A support assembly for supporting the body of auser in a chair, the support assembly comprising: a frame member havingan central opening therethrough and a channel formed around theperimeter thereof, the channel including a groove formed in a topsurface of the frame member and a plurality of spaced apart slotsextending through the frame member from a bottom wall of the groove; acarrier member having a rim portion substantially the same size andshape as the groove and a plurality of spaced apart tabs extendingdownwardly from the rim portion, said tabs having a hook portionextending outwardly from a lower end thereof and the slots being ofsufficient size to receive the tabs, the rim portion fitting in thegroove and the tabs extending through the slots such that the hookportion thereof engages an underside of the frame member to secure thecarrier member thereto; and a membrane of elastic material having anedge portion adapted to be attached to the carrier member such that saidmembrane covers the central opening.
 79. The support assembly of claim78 wherein the support assembly defines a seat and said membrane ispre-stretched in a longitudinal direction between a forward and rearportion of the carrier member in the range of 6% to 9% of an unstretchedlength of the mat, and said membrane is pre-stretched in a lateraldirection between side portions of the carrier member in the range of 1%to 2% of an unstretched width of the mat.
 80. The support assembly ofclaim 78 wherein the edge portion of said membrane is welded to thecarrier member.
 81. The support assembly of claim 78 wherein the edgeportion of said membrane is attached to an insert adapted to fit in acavity formed in said carrier member.
 82. An exposed fabric having aweave pattern for supporting the body of a user in a chair, the fabriccomprising: a plurality of groups of parallel strands of multifilamentyarn, each group having at least two adjacent strands; and a pluralityof monofilaments arranged in generally perpendicular interlockingrelationship with the strands, said monofilaments weaving alternativelyabove and below adjacent strands in a group, and said groups being heldin place by a plurality of pairs of said monofilaments, themonofilaments in each pair crossing over between each group of strandsto thereby maintain the position of the groups.
 83. The fabric of claim82 wherein each of said groups comprises three strands of yarn.
 84. Thefabric of claim 82 wherein the monofilaments are made of an elastomericmaterial.
 85. The fabric of claim 84 wherein the monofilaments areprestretched across an opening in a frame member.
 86. The fabric ofclaim 84 wherein the monofilaments have a greater width than thicknessto increase the surface area in contact with a user sitting on thefabric.
 87. The fabric of claim 86 wherein the monofilaments have anelliptical cross-section.
 88. The fabric of claim 84 further comprisingan elastomeric monofilament attached to each strand of yarn.
 89. Thefabric of claim 82 wherein there are about 24-26 monofilaments per inchand about 7-10 strands per inch.
 90. The fabric of claim 82 wherein themonofilaments are oriented in a lateral warp direction of a frame memberof the chair and the strands are oriented in a longitudinal fillingdirection of said frame member.
 91. An exposed fabric having a weavepattern for supporting the body of a user in a chair, the fabriccomprising: a plurality of groups of parallel strands of multifilamentyarn oriented in a longitudinal filling direction of a frame member ofthe chair, each group having at least two adjacent strands; and aplurality of elastomeric monofilaments prestretched in a lateral warpdirection of the frame member and arranged in generally perpendicularinterlocking relationship with the strands, said monofilaments weavingalternatively above and below adjacent strands in a group, and saidgroups being held in place by a plurality of pairs of saidmonofilaments, the monofilaments in each pair crossing over between eachgroup of strands to thereby maintain the position of the groups.
 92. Thefabric of claim 91 wherein each of said groups comprises three strandsof yarn.
 93. The fabric of claim 91 wherein the monofilaments have anelliptical cross-section.
 94. The fabric of claim 91 further comprisingan elastomeric monofilament attached to each strand of yarn.
 95. Thefabric of claim 91 wherein there are about 24-26 monofilaments per inchand about 7-10 strands per inch.
 96. An exposed fabric for supportingthe body of a user in a chair, the fabric comprising: a plurality ofgenerally parallel monofilaments having a greater width than thicknessto increase the surface area in contact with a user sitting on thefabric and provide an attractive appearance of the fabric.
 97. Thefabric of claim 96 wherein the monofilaments have an ellipticalcross-section.
 98. The fabric of claim 96 wherein the monofilaments aremade of an elastomeric material.
 99. The fabric of claim 97 wherein themonofilaments are prestretched across an opening in a frame member. 100.The fabric of claim 96 further comprising a second plurality ofmonofilaments arranged in generally perpendicular interlockingrelationship with said first plurality of monofilanents, said secondmonofilaments weaving alternatively above and below adjacentmonofilaments of said first plurality of monofilaments.
 101. Anapparatus for controlling the tilt range of a chair and the resistanceto tilting of the chair, compising: an elastomeric torsion springmounted to an axle; a tilt rate adjustment mechanism including an outersleeve member mounted to the spring, a screw having an axis positionedgenerally tangentially relative to the sleeve member, a block memberthreadably attached to the screw, an arm extending radially outward fromthe sleeve and operably engaging the block member, a first gear attachedto one end of the screw, and a second gear meshing with the first gear,whereby said second gear is actuated to rotate the first gear and screw,thus moving the block member in a linear direction along the axis of thescrew to move the arm a desired radial amount and adjust the initialreturn torque of the spring; a forward tilt limiter mechanism includingan arm extending outwardly from the axle, said arm being operablyengageable with a pivot member which is rotatable between a forward tiltposition and a standard tilt position; and a rearward tilt limitermechanism including an arm extending outwardly from the axle, said armbeing operably engageable with a cam member which is rotatable to adesired maximum rearward tilting position.
 102. A seating structurecomprising: a backrest having a top and bottom and comprising a frame,wherein said frame comprises a pair of spaced apart side membersdefining an opening therebetween, each of said side members having alength; a flexible membrane extending between and attached to said sidemembers across said opening; and an armrest moveably mounted to each ofsaid side members, each of said armrests adjustably moveable along atleast a portion of said length of said side members between at least afirst position and a second position, wherein said first position iscloser to said bottom of said backrest than said second position. 103.The seating structure of claim 102 wherein said flexible membranecomprises an elastic material.
 104. The seating structure of claim 103wherein said elastic material comprises elastomeric monofilaments. 105.The seating structure of claim 104 wherein said membrane furthercomprises multifilament yarns.
 106. The seating structure of claim 102further comprising a back support member disposed in said opening ofsaid frame between said side members, said back support membercontacting said flexible membrane.
 107. The seating structure of claim106 wherein said back support member is moveably connected to said frameand is moveable between said top and bottom of said frame.
 108. Theseating structure of claim 102 wherein said side members each comprisean elongated opening extending along at least a portion of said lengththereof, wherein at least a portion of said armrest is inserted intosaid elongated opening.
 109. The seating structure of claim 108 whereinsaid side members each comprise a cavity communicating with saidelongated opening, wherein each of said armrests is releasably engagedwith said frame inside said cavity.
 110. The seating structure of claim109 wherein said armrests each comprise a guide member disposed in oneof said respective cavities.
 111. The seating structure of claim 109wherein each of said armrests comprises a latch mechanism engaged withsaid frame.
 112. The seating structure of claim 102 wherein saidarmrests extend forwardly from said side members.
 113. The seatingstructure of claim 112 wherein said armrests extend laterally outwardlyfrom said side members.
 114. The seating structure of claim 102 whereinsaid frame further comprises a top and bottom member connecting saidside members, wherein said membrane extends between and is attached tosaid top and bottom members.
 115. The seating structure of claim 102wherein said frame comprises a channel, and further comprising a carriermember attached to said membrane, wherein said carrier member isinserted into said channel.
 116. The seating structure of claim 115wherein said carrier member is molded with an edge portion of saidmembrane.
 117. The seating structure of claim 102 wherein said membranecomprises a woven material.
 118. A method of adjusting a seatingstructure comprising: providing a backrest having a top and bottom andcomprising a frame, wherein said frame comprises a pair of spaced apartside members defining an opening therebetween, each of said side membershaving a length, a flexible membrane extending between and attached tosaid side members across said opening, and an armrest moveably mountedto each of said side members; and moving at least one of said armrestsalong at least a portion of said length of said respective side memberbetween at least a first position and a second position, wherein saidfirst position is closer to said bottom of said backrest than saidsecond position.
 119. The method of claim 118 wherein said flexiblemembrane comprises an elastic material.
 120. The method of claim 118wherein said elastic material comprises elastomeric monofilaments. 121.The method of claim 120 wherein said membrane further comprisesmultifilament yarns.
 122. The method of claim 118 further comprisingproviding a back support member disposed in said opening of said framebetween said side members, said back support member contacting saidflexible membrane.
 123. The method of claim 122 further comprisingmoving said back support member between said top and bottom of saidbackrest.
 124. The method of claim 118 wherein said side members eachcomprise an elongated opening extending along at least a portion of saidlength thereof, wherein at least a portion of said armrest is insertedinto said elongated opening.
 125. The method of claim 124 wherein saidside members each comprise a cavity communicating with said elongatedopening, and further comprising releasably engaging said frame with saidarmrest inside said cavity.
 126. The method of claim 125 wherein saidarmrests each comprise a guide member disposed in said cavities andwherein said moving said at least one armrest along at least a portionof said length of said respective side member between at least a firstposition and a second position comprises moving a respective guidemember within said respective cavity.
 127. The method of claim 118wherein said armrests extend forwardly from said side members.
 128. Themethod of claim 127 wherein said armrests extend laterally outwardlyfrom said side members.
 129. The method of claim 118 wherein said framefurther comprises a top and bottom member connecting said side members,wherein said membrane extends between and is attached to said top andbottom members.
 130. The method of claim 118 wherein said framecomprises a channel, and further comprising a carrier member attached tosaid membrane, wherein said carrier member is inserted into saidchannel.
 131. The method of claim 118 wherein said carrier member ismolded with an edge portion of said membrane.